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More details of Khalid Masood emerge

British police are combing through "massive amounts of computer data," have searched more than 20 sites and have contacted thousands of witnesses in a vast operation to trace how Khalid Masood became radicalised and launched a deadly terrorist attack in Westminster on Wednesday.

It also emerged that three more people have been arrested as part of the inquiry, taking the total to 11, before one of them, a woman, was released on bail.

London's top counter-terrorism officer, Mark Rowley, said more "significant" arrests had been made over Wednesday's attack, which killed four people and the assailant.

The attacker, Khalid Masood, was born Adrian Russell Ajao in southern England in 1964. He was also known as Adrian Elms and "may also be known by a number of other names," police said.

The latest arrests were a man and a woman detained early Friday in Manchester. Police believe Masood acted alone but Rowley said police were trying to determine whether others "encouraged, supported or directed him."

Detectives have searched 21 properties in London, Brighton, Wales, Manchester and the central English city of Birmingham in one of Britain's biggest counter-terrorism operations in years.

Wednesday's attack was the deadliest in Britain since suicide bombers killed 52 commuters on London's transit system in July 2005.

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"We've seized 2,700 items from these searches, including massive amounts of computer data for us to work through," Rowley said.

Masood drove his car into crowds on Westminster Bridge before fatally stabbing a police officer Keith Palmer on Parliament grounds before he was shot dead by police.

An American man from Utah, a British retiree and British female school administrator were killed on the bridge, and Palmer was stabbed to death at Parliament.

The latest victim, a man who died in a hospital Thursday, was identified as 75-year-old Leslie Rhodes from south London.

More than 50 people of a dozen nationalities were wounded in the attack, 31 of whom required hospital treatment, including one Australian.

Trish Neis-Beer, an Australian permanant resident, and a German national who lives and works in the Adelaide Hills, was among those injured.

Attorney-General George Brandis told the Senate on Thursday she was receiving treatment in hospital and consular staff were providing assistance.

"Those affected include a real cross-section of ages from at least 12 nationalities," Rowley said.

"It's a poignant reminder, I think, that the impact of this attack on the capital will reach around the world."

Rowley said two police officers targeted in the attack have significant injuries. Two other people also remain in critical condition, one with life-threatening injuries.

The 52-year-old attacker was born in south-eastern England and had most recently been living in Birmingham. Police say Masood has had a string of convictions between 1983 and 2003 for offences including assault and possession of an offensive weapon.

UK Prime Minister Theresa May said Thursday that Masood was "investigated in relation to concerns about violent extremism" some years ago. But she called him "a peripheral figure."

The manager of a hotel in the beachside city of Brighton in southern England, where Masood stayed the night before the attack, said he seemed unusually outgoing and mentioned details about his family, including having a sick father.

"He was normal, in fact friendly, because we spent possibly five or 10 minutes talking to him about his background," Sabeur Toumi told Sky News.

Londoners continued to lay flowers and sign condolence books for the victims on Friday, as Jewish, Christian and Muslim leaders gathered in solidarity outside Westminster Abbey.

Chief Rabbi Ephraim Mirvis said terrorists "seek to divide us. Londoners are showing right now that we will always stand up with strength to confront terror and we will never be cowed by it."

Some security experts, meanwhile, criticised police procedures after newly published video showed confusion and delays as the prime minister was being rushed out of Parliament after the attack.

Rowley, Scotland Yard's counter-terrorism chief, said changes to Parliamentary security may be needed.